“For many many years, I always felt like no matter how much I trained I never had stamina, and so I would go to the doctor and say, ‘I don’t feel like I can get in shape,’” Williams said after being diagnosed in 2011.

In the years since her diagnosis, Venus has captured three singles titles, but has been unable to make it past the quarterfinals at any Grand Slam tournament.

This changed when the two meet in tournaments. The rivalry used to take place near the climax of a major, but Tuesday's match up will be the second straight time they've met before the semis.

Though calling it a rivalry might be a bit of a stretch. The sisters have always supported each other, and each seems more muted in celebration when her sister is on the other side of the net.

Serena's had the chance to celebrate more often, though. She currently leads the all sister series 15-11.

More than head-to-head bragging rights will be on the line in Flushing Meadows Tuesday. With a U.S. Open title, Serena would become just the second person ever to win all four majors in one year in the Open Era. (Video via CNN)